Chemical mechanical polishing (CMP) is a semiconductor wafer flattening and polishing process that combines chemical removal with mechanical buffing. It is used for polishing and flattening wafers after crystal growing, and for wafer planarization during the wafer fabrication process. CMP is a favored process because it can achieve global planarization across the entire wafer surface, can polish and remove all materials from the wafer, can work on multi-material surfaces, avoids the use of hazardous gasses, and is usually a low-cost process.
FIGS. 1A and 1B show an example of the effect of performing CMP. In FIG. 1A, a semiconductor wafer 102 has a patterned dielectric layer 104, over which a metal layer 106 has been deposited. The metal layer 106 has a rough top surface, and there is more metal than necessary. Therefore, CMP is performed, resulting in FIG. 1B. In FIG. 1B, the metal layer 106 has been polished down so that it only fills the gaps within the dielectric layer 104.
FIG. 2 shows an example of a CMP system 200 for polishing the wafer 102 of FIGS. 1A and 1B. The wafer 102, with its dielectric layer 104 and metal layer 106, is placed on a platen 202 connected to a rotatable rod 206. A polishing pad 204 is lowered over the wafer 102, specifically over the metal layer 106 thereof. The polishing pad 204 is also connected to a rotatable rod 206. Slurry 210 is introduced between the polishing pad 204 and the metal layer 106, and the polishing pad 204 is lowered, pressured against the metal layer 106, and rotated to polish away the excess, undesired metal from the metal layer 106. The platen 202 is rotated in the opposite direction. The combined actions of the two rotations and the abrasive slurry 210 polish the wafer surface.
The polishing pad 204 can be made of cast polyurethane foam with fillers, polyurethane impregnated felts, or other materials with desired properties. Important pad properties include porosity, compressibility, and hardness. Porosity, usually measured as the specific gravity of the material, governs the pad's ability to deliver slurry in its pores and remove material with the pore walls. Compressibility and hardness relate to the pad's ability to conform to the initial surface irregularities. Generally, the harder the pad is, the more global the planarization is. Softer pads tend to contact both the high and low spots, causing non-planar polishing. Another approach is to use flexible polishing heads that allow more conformity to the initial wafer surface.
The slurry 210 has a chemistry that is complex, due to its dual role. On the mechanical side, the slurry is carrying abrasives. Small pieces of silica are used for oxide polishing. Alumina is a standard for metals. Abrasive diameters are usually kept to 10-300 nanometers (nm) in size, to achieve polishing, as opposed to grinding, which uses larger diameter abrasives but causes more surface damage. On the chemical side, the etchant may be potassium hydroxide or ammonium hydroxide, for silicon or silicon dioxide, respectively. For metals such as copper, reactions usually start with an oxidation of the metal from the water in the slurry. Various additives may be found in slurries, to balance their pH, to establish wanted flow characteristics, and for other reasons.
FIG. 3 shows a CMP head assembly 300 that can act as the head assembly which rotates the polishing pad 204 of FIG. 2. The CMP head assembly 300 is specifically one that is manufactured by and available from the Lam Research Corp., of Fremont, Calif. A carrier head latch 302 is secured over a head assembly 304, which is connected to an outer assembly 306 via a nut 307 and a cell 309. A gimbal assembly made up of a gimbal post 308, a pivot head plate 310, and a gimbal hub 312 reside within an inner manifold 314 of a manifold that also includes an outer manifold 316. The outer manifold 316 is situated over the polishing head 318, and a carrier film 322 is secured to the polishing head 318 via a retaining ring 320.
FIGS. 4 and 5 show in more detail, as perspective and cross-sectional views, respectively, the gimbal assembly of FIG. 3 that includes the gimbal post 308, the pivot head plate 310, and the gimbal hub 312. The gimbal assembly permits the polishing head 318 to incline freely in all directions. This is because the gimbal post 308 is situated over the pivot head plate 310, and more specifically a ball of the pivot head plate 310. To ensure that the gimbal assembly allows this free movement of the polishing head 318, the base of the pivot head plate 310 is meant to securely and precisely plug into the gimbal hub 312, with the raised outer lip of the base of the pivot head plate 310 in particular securely and precisely positioned within the bottom of the interior cavity of the gimbal hub 312. That is, the raised outer lip of the base of the pivot head plate 310 is meant to ensure that the pivot head plate 310 cannot become loose once it is plugged into the gimbal hub 312.
However, this design of the gimbal assembly of FIGS. 4 and 5 is problematic. At least occasionally the base of the pivot head plate 310 is too tight to force into the interior cavity of the gimbal hub 312. This is because the base of the pivot head plate 310 must precisely fit within the interior cavity of the gimbal hub 312 to ensure that there is a secure fit between the two. However, occasionally the base of the pivot head plate 310 may be just wide enough that it cannot be forced into the interior cavity of the gimbal hub 312. As a result, the pivot head plate 310 may tilt within the gimbal hub 312, decreasing polishing uniformity of the CMP tool. Furthermore, even if there is a good, secure, and precise fit of the pivot head plate 310 in the gimbal hub 312 initially, over time the pivot head plate 310 may shake loose, since nothing is securing the pivot head plate 310 to the gimbal hub 312. This also decreases polishing uniformity of the CMP tool.
Therefore, there is a need for a gimbal assembly that overcomes these problems. With respect to gimbal assemblies generally, there is a need for such assemblies that ensure that the pivot head plate cannot shake loose from the gimbal hub. With respect to gimbal assemblies within semiconductor fabrication tools, such as CMP tools, there is a need for such assemblies that ensure polishing uniformity and thus proper semiconductor fabrication. For these and other reasons, there is a need for the present invention.